As a humidity sensor or a moisture detecting element that makes use of the change of resistivity of a humidity sensitive material with change in humidity, there have been used those comprising lithium chloride, or these comprising ceramics of the system such as TiO.sub.2 -SnO.sub.2, TiO.sub.2 -V.sub.2 O.sub.5, MgCr.sub.2 O.sub.4 or ZnO-Li.sub.2 O-V.sub.2 O.sub.5. Among them, the humidity sensors comprising the above ceramics, or humidity sensitive ceramic sensors possess excellent stability in characteristics as compared with the sensors of lithium chloride. The ceramic sensors possess poor aging in the humidity detecting function as well as the sensors of lithium chloride. For example, when the ceramic sensors of the prior art are allowed to stand 3 months in an atmosphere of 40% relative humidity, the resistance thereof change by some 40% from the value of initial resistance. However, their function can be recovered to the previous level by heating since the ceramics used as a material for humidity sensors do not change in quality even if heated to high temperatures. For this reason, the ceramic sensors are combined with heating element for practical purposes to recover its humidity detecting function by intermittent heating. However, this lead to increase of the consumption of electric power.